Pyridine-2,3-dicarboxylate derivatives are useful intermediates for the preparation of herbicidal 2-(2-imidazolin-2-yl)nicotinic acids, esters, and salts. Several such herbicidal compounds are described in U.S. Pat. No. 5,334,576 and U.S. Pat. No. 4,798,619, which are incorporated by reference herein. A number of processes for the manufacture of pyridine-2,3-dicarboxylate derivatives, and their intermediates, have been described previously. For example, U.S. Pat. No. 4,723,011 provides a method for preparing pyridine-2,3-dicarboxylic acid esters by reacting an x-halo-p-ketoester with an α,β-unsaturated aldehyde or ketone in the presence of an ammonium salt. U.S. Pat. No. 4,816,588 provides a method for converting 8-substituted quinolines into pyridine-2,3-dicarboxylic acid esters by batch oxidation with large stoichiometric excesses of hydrogen peroxide and base. U.S. Pat. No. 5,614,635 provides a method for the preparation of pyridine-2,3-dicarboxylic acid esters by continuous oxidation of substituted quinolines with a large stoichiometric excess of hydrogen peroxide and base. The methods provided by these patents and others in the art have been criticized as being plagued with the problems of low yield and low purity, and the use of unstable halogenated oxalacetate intermediates.
U.S. Pat. No. 6,080,867 and U.S. Pat. No. 5,925,764, both of which are incorporated by reference in their entirety herein, disclose methods of preparing pyridine-2,3-dicarboxylic acid esters that purports to solve the problems described above. According to one method, an amino alkoxy (or alkylthio)oxalacetate is reacted with an α,β-unsaturated ketone in the presence of a solvent and an ammonia source. According to a second method, an amino alkoxy (or alkylthio)maleate or fumarate is reacted with an α,β-unsaturated ketone in the presence of a solvent.
While these methods overcome some of the problems of the earlier synthesis methods, pyridine-2,3-dicarboxylic acid esters manufactured according to this process, and their corresponding diacids, still contain impurities that affect the quality and processing behavior of process streams, product streams, and effluent streams. In particular, when the above-described method has been implemented for full-scale manufacturing of pyridine-2,3-dicarboxylic acid analogs, such as 5-ethyl-pyridine-2,3-dicarboxylic acid, product quality issues have been observed. Especially noted quality concerns include problems with product purity, color, and odor, and problems resulting from the formation of dark tars in process waste streams. As a direct result of these product quality problems, extra processing costs must be expended to dry filter product, develop procedures to remove impurities from below-specification diester and diacid, and clean tars from effluent treatment systems.
In light of the aforementioned problems, there remains a need in the art for an improved process for the manufacture of pyridine-2,3-dicarboxylic acid wherein the impurities are removed during the manufacturing process. Such an improved process would provide an improved diacid product and reduce manufacturing costs that are unnecessarily elevated due to the requirement of removing impurities from product streams and effluent streams.